This invention relates to elongated weak-link supercurrent devices and, more particularly, to controlling the propagation of mobile flux vortices in such devices.
In U.S. Pat. No. 3,676,718 granted to P. W. Anderson, R. C. Dynes and T. A. Fulton on July 11, 1972, there are described a variety of weak-link supercurrent logic devices which are capable of sustaining one or more trapped magnetic field (flux) vortices. In an extended Josephson junction (SIS) device, that is, one which is long in the x-direction compared to the Josephson penetration depth .lambda..sub.J, the patent teaches that a vortex is induced by a spatial variation of the supercurrent J(x,y) in which a positive supercurrent flows through the I-layer and into the contiguous superconductor to a depth of about .lambda..sub.L, the London penetration depth, then along the superconductor a distance of about 2.lambda..sub.J, thence through the I-layer again as a negative supercurrent into the opposite superconductor to a depth of about .lambda..sub.L and finally back to the point of beginning. Such a vortex supports a net magnetic flux of precisely .PHI..sub.o = 2.07 .times. 10.sup.-.sup.15 Wb, the well-known flux quantum. As defined in the patent, the term vortex means an entity which includes both the circulating supercurrent J(x,y) and the flux quantum .PHI..sub.o induced thereby.
Once created, the patent states, a vortex prefers to position and distribute itself in a region so that a local minimum of the sum of the magnetic energy plus the Josephson coupling energy is established. Where a plurality of such preferred locations are present in a single weak-link structure, it is possible to move the vortex from one such location to another by applying a force thereto as, for example, by applying a local current or magnetic field to a region near to the vortex.
In contrast, if the structure in which the vortex is created has no local minima of energy over an extended length in the direction of propagation (x-direction), then once set in motion the vortex will propagate at a velocity, and to a distance, determined by damping processes (e.g., single particle tunneling). This kind of structure could function as a transmission line on which information is carried in the form of a plurality of sequential vortices. At some point along the line it might be desirable to exercise various forms of control over the vortices. For example, to overcome damping processes in the line, it would be desirable to accelerate vortices which have slowed down. To perform logic functions, on the other hand, it might be necessary to switch selected vortices from one transmission path to another. Yet another application arises where the transmission line forms the vortex storage medium of a circulating memory. In the latter case, control is exercised both at the input, where vortices are introduced, and at some point in the transmission line to maintain constant the velocity and/or spacing of the vortices.